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Neurovascular Decompression in Cranial Nerves V, VII, IX, and X
Introduction
The pathobiology of vascular compression of the trigeminal (V), facial (VII), glossopharyngeal (IX), and vagus (X) nerves underlies a number of related neurologic disorders. Dandy (1932) first described vascular compression of the trigeminal nerve in the posterior fossa as a potential cause of trigeminal neuralgia, termed tic douloureux . Several decades later, Gardner and Sava (1962) identified vascular compressive lesions of the facial nerve as the likely underlying etiology of hemifacial spasm. Despite these early findings, it was not until Jannetta (1967) used the microscope to systematically study vascular compression of cranial nerves at their entry to or exit from the brainstem, in cadaveric studies and surgical case series, that the pathology and clinical findings associated with neurovascular compression was better understood.
Based on these findings, Jannetta developed a surgical technique to reliably relieve the vascular compression by displacing and insulating the offending vessels from the affected cranial nerves at their entry/exit zone to the brainstem without sacrificing neural integrity, termed microvascular decompression (MVD). , We define features associated with specific vascular compression syndromes of the cranial nerves, the surgical decompression techniques to effectively mitigate these vascular compression syndromes and the outcomes following surgical intervention.
Trigeminal Neuralgia
Clinical Features
General
Trigeminal neuralgia is the most common cranial nerve vascular compression syndrome with a prevalence of 20 to 200 per 100,000 (annual incidence of 4 per 100,000). It affects women more frequently than men (ratio, 2:1), and the peak age of onset is typically between 50 and 70 years of age (mean age, 63 years). The most common affected branches of the trigeminal nerve are V 2 and/or V 3 . Unilateral pain is most common but 1% of patients can have bilateral pain. Symptoms progressively worsen over the time with an increase in the frequency and severity of pain with a corresponding reduction in the duration of pain-free periods. Patients can develop sensory disturbances in the distribution of the affected branch. ,
Clinical Presentation
The pain associated with trigeminal neuralgia is stereotypical. It is classically described as a paroxysmal, intense, intermittent pain across the sensory distribution of one or more branches of the trigeminal nerve. This typically occurs in a V 2 and/or V 3 distribution, infrequently involving V 1 (only 2% of cases). The pain is described as an intense stabbing or electrical shock-like sensation, lasting seconds to minutes. It is most often triggered by light cutaneous stimuli. Patients typically will guard their faces and avoid touch, as well as shaving, washing, chewing, and brushing their teeth to avoid provocation of an attack. The pain is often more pronounced during the day. Between these episodic attacks, patients are characteristically free of pain.
Differential Diagnosis
Multiple Sclerosis
Paroxysmal attacks of trigeminal facial pain (similar in character to trigeminal neuralgia) can be caused by multiple sclerosis. Of patients with trigeminal neuralgia, 2% to 4% are found to have multiple sclerosis, including 18% of patients with bilateral trigeminal neuralgia. Of patients with multiple sclerosis, 1% to 3% will experience trigeminal neuralgia. Trigeminal neuralgia in multiple sclerosis occurs secondary to demyelination of the nerve at the root entry zone rather than vascular compression. MVD offers no benefit in this patient population. Rather, a neural destructive option can be considered in medically refractory patients.
Other Diagnoses
Although other facial pain syndromes do not resemble the clinical sudden, intense, paroxysmal nature of trigeminal neuralgia, they can often be misdiagnosed as trigeminal neuralgia. These include isolated orbital pain syndromes (slow onset with a prolonged time course over hours to days), cluster headache (pain has an insidious onset and is associated with runny nose, ptosis or a watery eye), post-herpetic neuralgia (constant pain with vesicles), dental disease (site specific), temporomandibular joint dysfunction, temporal arteritis (often with tenderness over superficial temporal artery) and post-traumatic neuralgia.
Imaging
Once the clinical diagnosis of trigeminal neuralgia is made, high-resolution magnetic resonance imaging (MRI) of the posterior fossa should be performed to rule out other differential diagnoses for trigeminal neuralgia, including demyelinating disease, infection, inflammatory processes, vascular malformations, and neoplasia. Adequate diagnostic workup is essential, as these other sources of atypical facial pain do not receive benefit from MVD.
Medical Management
First-Line Medications
First-line medical treatment of trigeminal neuralgia consists of carbamazepine or oxcarbazepine. , The mechanism of action of these drugs is thought to occur via blockade of voltage-sensitive sodium channels, quelling hyperexcited neurons. While carbamazepine has a long history of demonstrated efficacy, it can be limited by a significant side-effect profile, and oxcarbazepine may be better tolerated. A trial of 100 mg of carbamazepine given twice a day is a standard starting dose. Dosing is increased by 100 mg every other day until pain control is achieved or toxicity develops. Typical maintenance doses range from 300 to 800 mg/day divided over several daily doses. Adequate pain control can be achieved in the majority of patients with this regimen. ,
Dose limiting side effects of carbamazepine include nausea, vomiting, drowsiness, tachycardia, and hypotension. More severe toxicity may manifest as dysarthria, ataxia, hallucinations, tremors, seizures, central nervous system depression, or blood dyscrasias, including pancytopenia, aplastic anemia, and agranulocytosis. Given its improved side-effect profile, oxcarbazepine also may be used as a first-line agent. Oxcarbazepine is typically started at a dose of 150 mg twice daily and can be increased by 300 mg every 3 days until pain relief is obtained. Maintenance doses typically range from 300 to 600 mg twice daily.
Other Medications
Second-line medications may be used following carbamazepine or oxcarbazepine failure. Baclofen, a GABA B receptor agonist, depresses excitatory neuronal activity. Baclofen dosing for this indication ranges from 10 to 80 mg/day. A voltage-sensitive sodium channel inhibitor, lamotrigine has established efficacy in reducing trigeminal neuralgia pain compared to placebo. An initial dose of 25 mg/day is slowly titrated up to a range of 200 to 400 mg/day to achieve pain control. Other second-line agents include phenytoin, clonazepam, gabapentin, pregabalin, topiramate, levetiracetam, and valproate, all of which have shown improvement in trigeminal neuralgia pain compared to controls. ,
Surgical Indications
Trigeminal neuralgia patients that fail medical management (inadequate pain control or dose limiting side-effects) are candidates for surgical intervention. Surgical interventions include MVD, selective percutaneous lesioning and stereotactic radiosurgery ( Table 116.1 ). Each treatment modality carries certain risks and benefits. MVD is nondestructive and limits the risk of numbness, corneal anesthesia, and dysesthetic pain compared to other procedures. Additionally, MVD treats the underlying cause and is often curative. However, MVD is more invasive and has a higher risk of serious complications than the other treatment options. MVD is regarded as the procedure of choice for the treatment of trigeminal neuralgia in otherwise healthy (generally younger than 75 years) patients.
Table 116.1
Available Treatments for Medically Refractory Trigeminal Neuralgia
| Procedure | Benefits | Drawbacks |
|---|---|---|
| Percutaneous trigeminal neurolysis |
|
|
| Stereotactic radiosurgery |
|
|
| Micro vascular decompression |
|
|
Positioning
Positioning for MVD is most often performed in a lateral or park bench position ( Fig. 116.1 ). Following anesthetic induction, the patient is intubated, and the head is secured in three-point pin fixation. The patient is placed in the lateral decubitus position with the affected side facing up, and an axillary gel roll is placed on the dependent side. All pressure points are padded, and the nondependent arm is supported by a padded armrest. The neck is then secured in a slightly flexed position, and the head is rotated 10 to 20 degrees away from the affected side. The vertex of the head is maintained parallel to the floor. The patient is then secured to the operative table with 3-inch tape and/or Velcro straps. To provide additional working space, the upward-facing shoulder can be gently taped caudally.
Operative Technique
Following intubation and positioning, the hair is shaved over the incision site on the affected side, and sterile preparation and draping are performed. A vertical linear incision, approximately 6 cm long, is placed 5 mm medial to the mastoid notch and is centered two-thirds above the notch and one-third below ( Fig. 116.2 ). The scalp incision site is infiltrated with 0.25% Marcaine with epinephrine (1:200,000). The skin is incised, and hemostasis is achieved with bipolar electrocautery. Electrocautery is used to divide the occipital muscle mass down to the occipital bone. The anastomosis between the occipital and posterior auricular branches of the external carotid artery is often encountered with this exposure and is divided. The occipital muscle is stripped in the subperiosteal plane of the calvaria. Excessive lateral muscle dissection is avoided as this influences the exposure provided by the self-retaining retractor. Bridging emissary veins are coagulated, and their bony channels sealed with bone wax.
A Weitlaner (Integrated Medical Systems, Birmingham, AL) or cerebellar retractor is used to oppose the wound edges to provide adequate exposure. A single bur hole is placed at the transverse-sigmoid junction and enlarged into a circular craniectomy approximately 2.5 cm in diameter (see Fig. 116.2 ). The craniectomy should extend superiorly to the inferior portion of the transverse sinus and laterally to expose the medial portion of the sigmoid sinus. The lateral extension of the craniectomy often opens into the mastoid air cells, which are thoroughly waxed at the completion of the craniectomy and prior to closure. Care is taken to dissect the dura free from the inner table of the skull to avoid entering the dura or venous sinuses. Bridging veins encountered at this stage are coagulated.
The dura is opened in an inverted L-shaped manner 3 to 5 mm parallel to the sigmoid and transverse sinuses (see Fig. 116.2 ). The dura may be opened in a T-shaped manner at the superior corner if increased superior and/or lateral exposure is needed. The dura is secured with tenting sutures superiorly and laterally to retract the sinuses slightly and complete the exposure. Adhesions or bridging vessels encountered along the superior posterior margin of the cerebellum along the transverse sinus are bipolar coagulated and ligated to free the cerebellum. This expands exposure of the cerebellopontine angle and permits a wider working corridor to the site of neurovascular compression in the region of the brainstem and trigeminal nerve.
A self-retaining retractor system is used to provide gentle cerebellar retraction and exposure of the cranial nerves. Several retractors have been described for this purpose, including the Leyla V. Mueller and Company (Chicago, IL), Greenberg (Johnson and Johnson Healthcare Systems, Piscataway, NJ), Apfelbaum (Integra Life Sciences, Plainsboro, NJ) and modified Weitlaner (Integrated Medical Systems) retractor systems. Regardless of the system used, the retractor arm is positioned with a gentle arch and arm tension, allowing for adjustments and repositioning without undue force. Once the retractor blade is in place, the superior lateral margin of the cerebellum is retracted in an inferomedial direction, and the operating microscope is brought into the operative field. Additional medial retraction of the cerebellum can be used to increase exposure.
Once the lateral extent of the cerebellum is visualized, the angle of the approach follows the petrous bone anteriorly. The petrosal vein is identified and is typically encountered two-thirds of the distance between the dura and the trigeminal nerve. Great variability exists, and the vein may be close to the nerve or absent altogether. Most frequently, this vein is composed of two channels that join just before entering the dura. To facilitate adequate retraction and deeper dissection, the petrosal vein may need to be coagulated and divided sharply. Once this vein has been addressed, the retractor is advanced, keeping the blade close to the tentorium. Any significant degree of hemorrhage during cerebellar retraction should raise the suspicion of a torn dorsal bridging vein from the cerebellum to the tentorium. If this should occur, removing the retractor and slightly depressing the cerebellum allows visualization of this area.
Advancing over the superior surface of the cerebellum allows adequate exposure of the arachnoid overlying the trigeminal nerve and avoids erroneous exposure of the seventh and eighth nerves. When possible, the arachnoid surrounding the petrosal vein is opened initially to inspect potential venous compression of the trigeminal nerve prior to petrosal vein ligation, which would otherwise lead to an erroneous assumption of negative exploration. Thorough evaluation of the venous anatomy allows for the potential preservation of a portion of the petrosal system, while also providing adequate surgical access. While petrosal vein ligation is often well tolerated, preservation of part or all of the petrosal venous system reduces the risk of venous cerebellar infarction.
The arachnoid overlying the trigeminal nerve is opened to expose the nerve and surrounding structures ( Fig. 116.3 ). In some cases, the arachnoid is quite thin and can be easily punctured and opened with forceps or a nerve hook. In other cases, the arachnoid is thick and opaque and must be sharply dissected with microsurgical scissors. Throughout the arachnoid dissection, care is taken to avoid tugging on underlying structures. The fourth nerve is a thin, delicate structure within the arachnoid superior to the fifth nerve and just inferior to the tentorium. Sharp, rather than blunt, dissection of the arachnoid is used to avoid injuring this nerve. Once the arachnoid has been widely opened, the trigeminal nerve is easily identifiable, and the neurovascular relationships are assessed.
The pathologic neurovascular association tends to occur at the brainstem, and vessels impinging distally on the nerve are not typically the underlying cause of the neuralgia. Most frequently, the superior cerebellar artery (SCA) lies medial to the trigeminal nerve, loops down anterior to the nerve, and then emerges dorsally at the exit site of the nerve from the brainstem. Full arachnoid opening allows inspection of the entire circumference of the nerve at the brainstem. Importantly, the first vessel that is identified may not be the only compressive vascular channel, as multiple offending vessels have been described in a significant number of cases. It is also necessary to open the arachnoid anterior to the nerve to allow proper placement of the prosthesis.
Jannetta microsurgical instruments (V. Mueller and Company), Rhoton dissectors (V. Mueller and Company), or Apfelbaum dissectors (Integra Life Sciences) are of sufficient length and properly fashioned to allow adequate visualization throughout the operation. Various microsurgical scissors, including the Kurze left and right pistol-grip scissors (V. Mueller and Company) and straight and angled bayoneted microscissors, are also employed. Once the arachnoid is opened and the area thoroughly inspected, the exact nature of the compression can be determined. A microdental mirror (warmed in hot saline to reduce fogging) or an angled endoscope may be useful to inspect anterior to the nerve.
Identified compressive vascular loop(s) are dissected free of the trigeminal nerve. When the SCA is identified as the compressive vessel, the primary goal is to elevate the artery to a horizontal, rather than vertical, orientation and to displace it away from the nerve (see Fig. 116.3 ). The SCA may give off small perforating branches that supply the brainstem, although these branches normally do not hinder vessel elevation, provided their position is carefully preserved. Venous channels adjacent to the nerve are dissected free from the nerve and are coagulated and divided with the use of small up- and down-angled bipolar forceps that prevent the spread of current to the adjacent neural structures. When the offending vessel is located inferiorly to the nerve, it must be displaced further inferiorly away from the nerve. Most importantly, during decompression, kinking of the arterial channels as they are repositioned is avoided to prevent secondary ischemic injury.
To secure and insulate these vessels from the nerve, a small prosthesis (Teflon felt) is placed between the offending vessel and the nerve. Before placement, the felt is prepared by grasping and tearing it with two hemostats to create a soft prosthesis, resembling a cotton ball or pledget. This prosthesis is gently interposed between the nerve and the artery to ensure decompression (see Fig. 116.3 ). At times, venous channels may be the sole source of compression. In these cases, the offending veins are coagulated and divided. Coagulation alone simply shrinks these vessels, increasing tension on the nerve and allowing for potential recanalization. Therefore, it is important that the coagulated vessels are transected.
If surgical manipulation induces visible spasm, Gelfoam soaked in topical papaverine is placed on the vessel until the spasm reverses. Attention is then directed to closure. The operative field is irrigated with warm saline, and the retractor is removed. The cerebellum is inspected to ensure there is no surface bleeding. Interrupted or running 4-0 braided sutures are used to form a watertight dural closure that can be supplemented with fibrin glue. Exposed mastoid air cells are covered with bone wax. The bone flap can then be fixed in place with cranial fixation hardware or titanium mesh can be used to cover the bony defect. The wound is then closed in layers using resorbable sutures, followed by staples in the skin.
Postoperative Care
Patients are observed carefully for any neurologic decline or signs of posterior fossa pathology due to either cerebellar swelling or hematoma. Patients are allowed out of bed, begin oral intake, and initiate pharmacologic deep venous thrombosis prophylaxis on the first postoperative day. The urinary catheter is removed at that time and intravenous fluids are discontinued as soon as the patient is able to achieve adequate oral intake. The surgical dressing is removed on the second postoperative day.
Patients may have a postoperative headache, which can typically be controlled with mild narcotic analgesics. This usually subsides within the first 2 days but may occasionally persist for several weeks and is treated symptomatically. Most patients recover rapidly from this procedure and are ready for discharge by the second or third postoperative day.
Operative Results
Operative Findings
An artery is found compressing the nerve in 80% to 90% of trigeminal neuralgia cases ( Table 116.2 ). The most common artery found compressing the trigeminal nerve is the SCA. In 5% to 10% of cases, venous compression or anterior inferior cerebellar artery (AICA) may be identified as the underlying etiology. Less than 5% of the time, no vascular lesion will be identified.
Table 116.2
Offending Lesion in Microvascular Decompression of Trigeminal Nerve
| Structure | Frequency |
|---|---|
| Arterial channels SCA AICA SCA and AICA Other a |
80%–90% 75%–80% 5%–10% 5%–10% Less than 1% |
| Venous channels | 5%–10% |
| No finding | Less than 5% |
AICA, Anterior inferior cerebellar artery; SCA, superior cerebellar artery.
a Other offending arteries include ectatic basilar and vertebral arteries.
Outcomes
Rates of initial pain control are excellent after MVD (80% to 97% of patients; Table 116.3 ). , While the vast majority of patients wake from anesthesia without neuralgia pain, it can take up to a month before pain relief is achieved. Preoperative medical management regimens are continued following surgery and weaned over weeks to months as tolerated. Long-term rates of successful pain control following MVD are excellent (70% to 80% of patients are pain free 5 to 15 years following surgery). , , , Medical management is infrequently needed long term but can be used to augment pain control.
Table 116.3
Microvascular Decompression for Trigeminal Neuralgia Results
| Result | Frequency |
|---|---|
| First-time MVD | |
| Immediate | 80%–97% |
| 1 year | 85%–90% |
| 5 years | 75%–80% |
| 15 years | 70%–75% |
| Repeat MVD | |
| Immediate | 90%–93% |
| 1 year | 60%–70% |
| 10 years | 40%–45% |
MVD, Microvascular decompression.
Factors Associated with Outcome
Studies have identified factors associated with post-operative pain relief. Arterial, rather than venous, compression correlates with improved pain control following MVD. , Similarly, patients with a greater degree of neurovascular compression identified during surgery are more likely to have relief after MVD. The pattern, quality and nature of preoperative pain also have a major impact on post-operative outcomes. Classic episodic, paroxysmal preoperative pain is associated with excellent outcome. Alternatively, patients whose pain includes a significant component of constant (greater than 50% overall) pain are less likely to receive benefit postoperatively. Patients with classic pain triggers are more likely to have a good outcome. Older patient age is also associated with good outcomes.
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